The present invention relates to an image reproducing or display arrangement for motor vehicles in which a rear image is reproduced as virtual image in the field of vision of the vehicle user by means of a lens and the windshield following in the path of the light rays.
Such arrangements, which are also known under the designation "Head-Up Display" are known, for example, from the EP No. 216 014 A and the DE No. 35 32 120 A. In these prior art arrangements, the generator for the real image and the lens are located within the area of the instrument panel. By reason of the spatially constricted installation conditions, this leads to considerable constructive problems. The latter are made more difficult if it becomes necessary to provide additional devices for the adaptation of the virtual image to the respective vehicle users or for the change of the intercept or image distance of this image located preferably at a large distance in front of the motor vehicle.
The present invention is concerned with the task to provide an image reproducing or display arrangement of the aforementioned type which can be accommodated in a constructively simple manner with smallest possible space requirement within the area of the instrument panel inside of the motor vehicle.
The underlying problems are solved by the present invention in that the real image and the lens are located at the upper edge of the windshield and in that a mirror is arranged between the lens and the windshield within the area of the surface of the instrument panel.
Within the area of the instrument panel, only the mirror is now provided which, for example, can be constructed as flat or plane body of slight thickness. Additionally, it is possible to minimize the space requirement of the mirror by its construction as holographic optical element--in this case the space requirement corresponds to that of a thin film--or as Fresnel mirror. The shape of a mirror element constructed in this manner can then be matched to the surface of the instrument panel and, for example, can then rest directly on the same.
The accommodation of the producer or generator for the real image and of the lens in the upper area of the windshield offers the advantage to be able to dispose over a relatively large installation space which heretofore has been used at most only for a sun visor. The possibility can also eventually be realized in this installation space to change the virtual image in the mentioned manner.
A further improvement of the present invention is achieved in that the mirror is constructed as collimator. It becomes possibly thereby, especially in conjunction with the lens, to produce a virtual image which lies at some distance in front of the vehicle and possibly even at infinity. An accommodation of the human eye is dispensed with thereby because the virtual image lies in a distance range in which the vehicle user watches the traffic.
Such a collimator mirror can also be constructed in the form of a holographic optical element or of a Fresnel mirror. The latter, analogous to the known Fresnel lens, is formed by ring-shaped partial sections which in cross section adjoin one another step-like.
For example, a liquid crystal (LCD) indicating device may serve as image producer or generator. Such an arrangement distinguishes itself by a small space requirement. It offers additionally the possibility to supply a variable light intensity with the aid of a secondary light source placed underneath the same. The light intensity can be matched to the outside brightness in order to realize always a sufficient contrast. For that purpose, the outside brightness can be determined with the aid of a known photodiode and can be converted into a corresponding control signal for the brightness adjustment of the secondary light source.
In the alternative thereto, the LCD illumination can also take place during the day by means of daylight which is picked up by a light collector and is conductor to the LCD by way of a light conductor.
The already mentioned means for the variation of the position or image distance of the virtual image can be realized in a constructively particularly simple manner with the aid of two convex lens elements which are adjustable in their mutual distance. The real image is thereby located in the focal plane of the first lens element as viewed in the beam path. Also, such lens elements can be constructed again as holographic optical elements or Fresnel lenses. By the variation of the mutual spacing of the two lenses from one another, the real intermediate image produced by the second lens is changed in its distance from the collimator. A change of the virtual image visible by the vehicle user also results therefrom as regards its size, respectively, its distance.
This representation of the virtual image at the distance and the means for the variation of the distance can be achieved constructively in a particularly advantageous and cost-favorable manner in that the image is produced by means of a liquid crystal film and/or by the use of a lens element including convex lenses, respectively, holographic optical elements changeable in their mutual spacing whereby the real image is in the focal plane of the first lens, respectively, of the first holographic optical element.